1. Field of the Invention
The present invention relates to chemical vapor deposition of films of ruthenium. More particularly, the invention relates to chemical vapor deposition of ruthenium and ruthenium oxide, and to new compositions useful in forming said depositions.
2. State of the Art
Use of chemical vapor deposition ("CVD") methods for depositing a thin film of material on a substrate, such as a silicon wafer or other exposed material surface on a wafer or other semiconductor base, are widely known and used in the semiconductor fabrication industry. In CVD, a precursor, such as a heat decomposable volatile compound, is contacted with a substrate which has been heated to a temperature above the decomposition temperature of the precursor. In this fashion, a coating, which typically consists of a metal, metal mixture or alloy, ceramic, metal compound, or a mixture thereof, depending on the choice of precursors and reaction conditions, is formed on the substrate.
Use of CVD as a thin film formation method includes numerous desirable characteristics, such as the ability to readily control the composition of the thin film and the ability to form a thin film without contamination of, or damage to, the substrate. CVD may also be used to deposit films of metals into vias, trenches, and other recesses or stepped structures. In situations where conformal thin-film deposition is required, CVD techniques are a preferred method of deposition, since evaporation and sputtering techniques cannot be used to form a conformal thin-film deposition layer.
While CVD techniques have been described in the literature with reference to many metals and metalloids, commercial use of CVD has been predominantly confined to deposition of a few metals and metal compounds, such as tungsten, silicon, titanium nitride, silicon oxide, iron, and aluminum. CVD of other metals has been limited due to a variety of reasons, including formation of poor film quality, requirement of high processing temperatures, lack of suitable precursor compounds, and instability of the precursors used in the deposition systems. The availability of suitable volatile and heat-decomposable precursor compounds appears to be the greatest limiting factor in the application of CVD to the production of metal-containing films.
In integrated circuit processing, selected precursor compounds have been used to form conducting films that can maintain their integrity at elevated temperatures. Ruthenium and ruthenium dioxide (RuO.sub.2) are particularly well-suited as conducting films for such applications since they have good electrical conductivities, exhibit high stability over a wide temperature range and exhibit good adherence to silicon, silicon dioxide, and to ceramic oxides. Films of ruthenium and ruthenium oxide deposited by CVD have been proposed to be useful for contact metallizations, diffusion barriers, and gate metallizations. M. L. Green et al., J. Electrochem. Soc., 132, 2677 (1985).
A number of CVD methods, utilizing a variety of ruthenium precursors, have been disclosed or used in the formation of ruthenium films with varying degrees of success. One such method involves a chemical spray deposition process wherein tris(acetylacetonate)ruthenium in butanol is converted into an aerosol spray using a hydrogen/nitrogen mixture as the carrier gas. Triruthenium dodecacarbonyl, ruthenocene, and tris(acetylacetonate)ruthenium have also been compared as CVD precursors in the formation of ruthenium and RuO.sub.2 by M. Green et al., in J. Electrochem. Soc., 132, 2677 (1985). However, because none of the aforementioned precursors are very volatile, high deposition rates using these precursors are difficult to obtain.
U.S. Pat. No. 4,250,210, issued Feb. 10, 1981 to Crosby et al., discloses the use of ruthenium 1,3 dione compounds, such as tris(acetylacetonate)ruthenium and its fluorinated derivatives, in the CVD of ruthenium films. Although the fluorinated ligands are said to provide greater volatility and good deposition rates when heated to over 200.degree. C., difficulties in attaining uniform coatings are noted due to the poor stability of the precursors. The low stability of the precursors yields inferior coatings that are especially pronounced when aged samples of the precursors are used. Furthermore, organic byproducts (e.g., oligomers of the acetylacetonate ligands) with very low vapor pressures are formed and collected in the reactor during the volatilization process, which can create a serious contamination problem in production-scale applications of the tris(acetylacetonate)ruthenium precursors.
Also disclosed in the Crosby patent is the use of ruthenium carbonyl chloride and penta(trifluorophosphine)ruthenium as precursors for ruthenium CVD. Use of these precursor compounds, however, is undesirable because the obtainable rates of deposition of ruthenium are very low. Additionally, ruthenium carbonyl chloride corrodes certain substrates, making a consistent product preparation difficult or impossible. This lack of consistency in the product can show up as a substantially nonvolatile form of the carbonyl chloride, which decomposes before it can volatilize.
U.S. Pat. No. 5,372,849 issued Dec. 13, 1994 to McCormick et al. discloses the use of organometallic precursors of iron, ruthenium, and osmium. Many of the disclosed ruthenium precursors are high volatility compounds that allow for high deposition rates and a reduction in carbon contamination in a non-reduced atmosphere. However, many of the disclosed ruthenium precursors are large or complex molecules that presumably exist in a predominantly solid state and which, due to their solid state, require sublimation for use in CVD of films of ruthenium.
Thus, in view of the described shortcomings of the available precursors, a continuing need exists for improved ruthenium precursors useful for the CVD of films of ruthenium. More specifically, a need exists for high-volatility ruthenium precursors that are easy to prepare and to use in low-temperature CVD processes and which are capable of depositing high quality, continuous films of ruthenium having good surface morphology.